Declaring internal entities in an xsl-file

I have an xsl-file that transforms an xml-file into xhtml. In this xsl-file
I want to specify some internal entities for  font face and font size, so
that I only need to change it at one place. Is there any way I can declare
just the entities without having a "real" dtd?

If not, how can I specify booth the xsl dtd and the xhtml dtd, and where can
I find these dtds?

- Mattias


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